1. Field of the Invention
This invention relates to a method, and means to perform the method, of testing materials formed of composite construction for structural damage and making instantaneous assessments of the extent of damage. More specifically, the invention relates to the use of optical fibers in a composite damage detection method.
2. Description of the Prior Art
Many types of composite materials may be formed in solid laminate or in various laminated core constructions.
Solid laminated composites are formed by a plurality of alternating cloth layers, or plys, which typically are impregnated with a pre-mixed resin system, there being many different types of cloth and many different types of resin used in many different combinations and employing many different procedures for ply build-up. Sections of components or complete components may include light in weight resin reinforced paper honeycomb cores, metal honeycomb cores, or various plastic foam cores.
All types of composite materials are strong but light in weight. Their strength compares favorably with aluminum, titanium alloys and other metals commonly used in fixed and rotary wing aircraft, and in space craft. Due to their high strength for weight ratio, they have become the material of choice in many airborne applications.
Accordingly, aircraft and space craft parts are now being manufactured in composite form. Many flights of airborne vehicles of composite construction have been flown, and a serious problem has become apparent. The problem is so serious that the composite industry has invested and is continuing to invest substantial amounts of money in an effort to overcome it, but the solutions found heretofore have been mere refinements of previous technology.
The problem that has been encountered relates to testing the composite materials for safety purposes. Namely, the known methods of testing composite materials for structural defects or other forms of damage are inadequate. The result has been loss of life and loss of highly sophisticated equipment.
For example, it has been publically reported that the tail section of a U.S. Air Force F-14 aircraft separated from the fuselage of the aircraft in flight, which structural failure resulted in the death of the crew members and in loss of the aircraft. The aircraft had been subjected to the known safety inspections, to no avail.
The F-14 disaster, unfortunately, is merely one of a series of disasters that have befallen aircraft having composite components.
The difficulty in safety checking a composite part arises because of the properties of the materials of which component parts are made. Being non-metallic, the customary techniques of examination are of no effect. More specifically, the parts cannot be effectively examined with X-rays and cannot be visually inspected with any degree of reliability. Composite components can be rendered structurally unsound by either formed-in improperly bonded defects, or by subsequent damage to a properly formed component. Since composite components are often highly stressed structural elements, the application of normal stress to a component with undetected formed-in defects or undetected subsequent damage can result in catastrophic failure of the component.
Due to the non-monolithic nature of composite construction, all but the most obvious of defects or damage are difficult to non-destructively detect, especially when the unsound area is deep inside the structure or in a particularly inaccessable location.
Presently, the same means and equipment are used to detect both defects and damage. All present means and methods are different forms of an acoustic process, i.e., inexact portable ultrasonic, C-scan (which is much more accurate), or McDonnell Douglas' new water jet ultrasonic inspection. All these methods are useful for detection of formed-in defects where equipment, technicians, set-up time, and component access is readily available.
However, all current test methods are inadequate in many ways for detection of subsequent damage to composite components when there is a critical need for accurate, immediate field information as to the serviceability of often inaccessible composite components.
Acoustical techniques are the industry standard and have met with some degree of success, but such techniques have serious limitations. Perhaps the greatest limitation is the need for complex, costly and immobile equipment to conduct the tests. Acoustical tests must be performed in an acoustics laboratory under carefully controlled conditions, and can not be performed in the field, without dismantling and removal of the component, with any acceptable degree of reliability at any location not suitably equipped. Moreover, due to the size and weight of the acoustical testing equipment, as well as the nature of the test process itself, such equipment cannot be carried by aircraft or spacecraft, even if the test process could be conducted in-flight, with the result that developing composite problems cannot be monitored by the crews of the vehicles.
Go or no-go decisions must often be made quickly in the field wherever composite aircraft are in use. The no go decision becomes virtually mandatory in most situations where the extent of damage to the aircraft is unknown, due to the very high degree of risk involved. Where unknown damage has occured and the aircraft is put in service, disasterous and expensive results can and have occured.
The art of composite material safety testing is so poorly developed that it is hampering governmental and private development of composite airborne vehicles and is endangering the lives of crews of composite-reliant aircraft.
There is a clear need for a testing method, and means for performing the method, that tests composite materials with a high degree of reliability and in remote locations with minimal amounts of equipment and unskilled personnel, but the state of the art contains no suggestions on how to fulfill this important need.
Another art, unrelated to the art of composite materials, is the art of fiber optics. Perhaps the greatest use of optical fibers is in the information transmission industry, where optical fibers are being used more and more in lieu of wires. Also, researchers having an interest in the computer industry are currently investigating the feasibility of the use of optical fibers in computers, in lieu of the electronics parts now used, in order to increase computing speed. Optical fibers are used in other applications as well, but it is believed that the disparate arts of composite materials and optical fibers were segregated prior to the filing of this disclosure.